In order to understand the mechanism of the interaction between chymotrypsin and its inhibitor - OMTKY3, it is necessary to relate the biochemical behavior of the mutants of OMTKY3 to their structural properties. pH titration by NMR can provide such information. By means of pH titration, we can obtain the pKa value of different amino acid residues. By comparison of these pKa values,, those of wild type and those of other mutants, we can obtain important structural information for OMTKY3 and arrive at a further understanding of its inhibitory action on chymotrypsin.